IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11h, 802.11n, 802.16, and 802.20, which are hereby incorporated by reference in their entirety, define several different standards for configuring wireless networks and devices. According to these standards, wireless network devices may be operated in either an infrastructure mode or an ad-hoc mode.
In the infrastructure mode, the wireless network devices or client stations communicate with each other through an access point. In the ad-hoc mode, the wireless network devices communicate directly with each other and do not employ an access point. The term client station or mobile station may not necessarily mean that a wireless network device is actually mobile. For example, a desktop computer that is not mobile may incorporate a wireless network device and operate as a mobile station or client station.
A wireless network that operates in the infrastructure mode includes an access point (AP) and at least one client station that communicates with the AP. For example, the wireless network may operate in an infrastructure mode. Since the client stations are often battery powered, it is important to minimize power consumption to preserve battery life. Therefore, some client stations implement a low power mode and an active, or “awake,” mode. During the active mode, the client station transmits and/or receives data. During the low power mode, the client station shuts down components and/or alters operation to conserve power. Usually, the client station is not able to transmit or receive data during the lower power mode.
Wireless network devices may be implemented by a system on chip (SOC) circuit that includes a baseband processor (BBP), a medium access controller (MAC) device, a host interface, and one or more processors. A host communicates with the wireless network device via the host interface. The SOC circuit may include a radio frequency (RF) transceiver or the RF transceiver may be located externally. The host interface may include a peripheral component interface (PCI) although other types of interfaces may be used.
A power management device controls and selects different operating modes of the client stations. During operation, the power management device instructs some of the modules to transition to a low power mode to conserve power. Additional information may be found in U.S. patent application Ser. Nos. 10/650,887, filed on Aug. 28, 2003, 10/665,252, filed on Sep. 19, 2003, and 11/070,481 filed on Mar. 2, 2005, which are hereby incorporated by reference in their entirety.
Referring now to FIG. 1, a first wireless network 10 is illustrated in an infrastructure mode as defined by IEEE 802.11 and other future wireless standards. The first wireless network 10 includes one or more client stations 12 and one or more access points (AP) 14. The client station 12 and the AP 14 transmit and receive wireless signals 16. The AP 14 is a node in a network 18. The network 18 may be a local area network (LAN), a wide area network (WAN), or another network configuration. The network 18 may include other nodes such as a server 20 and may be connected to a distributed communications system 22 such as the Internet.
The client station 12 does not continuously transmit data to or receive data from the AP 14. Therefore, the client station 12 implements a power savings mode when the client station 12 and the AP 14 do not have data to exchange. Data commonly remains intact in a network for a predetermined amount of time before it is dropped. The incorporated IEEE standards provide the opportunity for the client station 12 to inform the AP 14 when the client station 12 is entering a low power mode (and will not be capable of receiving data for a predetermined period). After notifying the AP 14, the client station 12 transitions to the low power mode. During the low power period, the AP 14 buffers data that is intended to be transmitted to the client station 12. Following the low power period, the client station 12 powers up and receives beacon transmissions from the AP 14. If the beacon transmissions indicate that the AP 14 has data for the client station 12, or the host processor of the client station 12 indicates it has data to transmit, the client station 12 remains active. Otherwise, the client station 12 enters the low power mode again.
The AP 14 attempts to transmit a beacon at a target beacon transmission time (TBTT). Before the AP 14 sends out a beacon transmission, the AP 14 determines whether other devices are currently transmitting data so that other devices are able to use the network. The client station 12 transitions to the active mode prior to a beacon transmission to queue frames to transmit to the AP 14 in a buffer. Immediately following a beacon transmission, the AP 14 can exchange frames with one or more client stations 12 in a deterministic order. For example, the AP 14 and the clients stations 12 may exchange data according to Time Division Multiplexed (TDM) protocol. The use of TDM protocol minimizes collisions that may occur when one or more of the client stations 12 attempt to transmit data to the AP 14 simultaneously. However, other wireless networks that are located near the first wireless network 10 may not operate according to the above-described TDM protocol. As such, collisions may occur between the other wireless networks and the first wireless network 10.
In another implementation, each client station 12 may wait for a random period prior to transmitting. This random period, or backoff period, reduces the likelihood that multiple client stations will attempt to transmit simultaneously. As such, a wireless network that implements the random backoff period has improved collision avoidance over a wireless network that implements a pure TDM scheme when multiple networks exist in an overlapping region. However, random backoff does not guarantee collision avoidance. In certain applications, collision avoidance is critical. For example, wireless networks that exchange multicast data typically do not include a positive acknowledgement feature. In other words, a transmitting station does not receive acknowledgement from a receiving station that the data was correctly received. Further, as a result of the random backoff periods, the time required for all stations in the wireless network to complete a set of frame exchanges is increased.
Referring now to FIG. 2, a second wireless network 24 operates in an ad-hoc mode. The second wireless network 24 includes multiple client stations 26-1, 26-2, and 26-3 that transmit and receive wireless signals 28. The client stations 26-1, 26-2, and 26-3 collectively form a LAN and communicate directly with each other. The client stations 26-1, 26-2, and 26-3 are not necessarily connected to another network. The client stations 26-1, 26-2, and 26-3 do not continuously transmit data to and receive data from each other. The client stations 26 implement a power savings mode when one of the client stations 26-1 does not have data to exchange with the other client stations 26-2 and 26-3.
The client stations 26-1, 26-2, and 26-3 are not required to buffer data as performed in the AP. For example, the client station 26-1 transmits the beacon to the other client stations 26-2 and 26-3. The client stations 26-2 and 26-3 transition to the active mode prior to the beacon transmission. During a beacon interval defined by the beacon transmission, each client station 26 transmits data in a deterministic order. For example, the client stations 26 may transmit data sequentially.